1. Field of the Invention
The present invention provides a projection display device, and more particularly, a projection display device for displaying electrically encoded images.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art projection display device 10. A light source 12 supplies light to a uniform illumination optical device 14 set in front of the light source 12 to convert light from light source 12 into a beam that has an even radiance and that is almost square. A dichroic device 16 splits the beam into three beams of different color (red, green, and blue). An approximately square trichromatic prism 18 having three input surfaces 18a and an output surface 18b combines the three different input beams to form an output beam. Three light modulation devices 20, made from liquid crystal panels, are set in front of the three input surfaces 18a of the trichromatic prism 18. The light modulation devices 20 are electrically controlled and modulate the input beams to produce modulated output beams having an image. Three focus lenses 17, 19 and 21 are set in front of the three light modulation devices 20 to focus the input beams from the dichroic device 16 onto the three light modulation devices 20. A projection lens is disposed in front of the output surface 18b of the trichromatic prism 18 to project the output light from the trichromatic prism 18 to a screen 24. The light modulation devices 20 are formed from transparent, monochromatic liquid crystal displays to display monochromatic images. The trichromatic prism 18 combines the three monochromatic images from three monochromatic liquid crystal displays into a chromatic image and outputs the chromatic image via the output surface 18b. 
A first dichroic mirror 26 within the trichromatic device 16 splits a monochromatic beam from the optical device 14. A reflector 27 reflects the monochromatic beam from the dichroic mirror 26 to the focus lens 17. A second dichroic mirror 28 splits the other two color beams out of the beam from the dichroic mirror 26. In this manner, the beam from the uniform illumination optical device 14 is first split into a red input beam by the first dichroic mirror 26, and delivered to the focus lens 17. The light is then split into a blue input beam by the second dichroic mirror 28 and directed to the focus lens 19, with the remaining green input beam directed to the focus lens 21. The green input beam passes through two lenses 30 and two reflectors 32 on its way to the focus lens 21.
It is clear from FIG. 1 that the red, blue, and green beams do not travel the same distance in the dichroic device 16. The red and the blue beams travel about the same distance, both of which pass through one dichroic lens and one reflector. But the traveling distance of the green input beam is longer than that of the red and the blue input beams, which affects the illumination of the green input beam. To compensate for the loss of illumination in the green input beam, the two lenses 30 are placed in front of the two reflectors 32 to focus the green input beam. Although this configuration balances the green input beam with the two other beams, it makes the projection display device 10 more complicated and increases overall manufacturing costs.
It is therefore a primary objective of the present invention to provide a projection display device that uses an L-type optical module to resolve differences in the traveling distances of the three color input beams, and to simplify the configuration of the prior art projection display device.
Briefly, this invention provides a projection display device to display electrically encoded images, and includes an L-type optical module to control the route of the beams. This optical module includes first, second, and third polarization beam splitter mirrors, and a first retarder film. The first and the third beam splitters are in the same plane, which is perpendicular to the second beam splitter. The first retarder film is located between the second and the third beam splitters. When a monochromatic polarized beam and a bi-chromatic polarized beam are input into the optical module, the monochromatic polarized beam will be directed to the first polarization beam splitter mirror and the bi-chromatic polarized beam will be directed to the third polarization beam splitter mirror. The first polarization beam splitter mirror directs the monochromatic polarized beam to the first modulation unit and the first modulation beam from the first modulation unit to the second polarization beam splitter mirror. The third polarization beam splitter mirror splits the bi-chromatic polarized beam into two monochromatic polarized beams according to their polarities, directing them to the second or the third modulation unit, and directs the second and the third modulation beams reflected from the second and the third modulation units to the second polarization beam splitter mirror via the first retarder film. The second polarization beam splitter mirror combines the first, the second, and the third modulation beams to form an output beam.
It is an advantage of the present invention that the traveling distances of three different colors of polarized beams are almost the same because of the arrangement of the optical module.
This, and other objectives of the present invention, will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.